Effect of Light on the Activity of Metallocene Catalysts in the Gas‐Phase Polymerisation of Ethylene

It was found that halogen light can have a strong effect on the polymerisation activity of silica‐supported metallocene catalysts. Three different metallocene catalysts have been tested in the gas‐phase polymerisation of ethylene. The effect of activation depends not only on the type of transition metal, but also on the wavelength and intensity of light. The activation by light was found to be a reversible process.     

Raman Optical Activity of a Purely σ‐Bonded Helical Chromophore: (−)‐(M)‐σ‐[4]Helicene

The recent synthesis of enantiomerically pure (−)‐(M)‐σ‐[4]helicene has provided an archetype helical model system for vibrational optical activity, comparable to what π‐helicenes represent in the field of electronic optical activity. We present the first measurements and the first calculations of the Raman optical activity (ROA) of this interesting molecule. Observed and calculated ROA is large throughout the vibrational spectrum, in agreement with expectations, and spectacular effects, with Δ values close to 0.5%, occur in the 900‐cm⁻¹ region. Agreement between the experimental spectrum and the theoretical one, calculated with density‐functional theory for the vibrational part and Hartree‐Fock linear response theory for the molecular electronic tensors, is excellent, clearly the best that has been achieved to date in the field. This allows us to place confidence in the results of an analysis of Raman and ROA scattering generation in the molecule, obtained by a newly developed graphical procedure for extracting this kind of information from ab initio calculations. One finds that relative contributions made by carbon and hydrogen atoms can be comparable in size, but can also vary considerably, even between closely lying vibrations, and that, for most vibrations, the generation of ROA difference intensity is distributed rather differently than that of Raman intensity over the shape of the molecule. The sign of the ROA is, for the set of vibrations in the 900‐cm⁻¹ region, which we analyze in detail, determined by coupling terms between the two halves of the molecule, while Raman intensity is primarily generated within the two fragments, with coupling terms between them only adding to or substracting from it.     

N‐Isobutylamides and Butyrolactone from the Fruits of Zanthoxylum integrifoliolum

Further investigations of the CHCl₃‐soluble fraction of the fruit of Zanthoxylum integrifoliolum led to the isolation of three new N‐isobutylamides: lanyuamide IV ( 1 ), lanyuamide V ( 2 ), and lanyuamide VI ( 3 ), along with lanyulactone ( 4 ), a new butyrolactone derivative. The structures of these new compounds were elucidated by spectroscopic data.     

Crystal Structure,Magnetic Exchange Interaction,and DFT Study of 2,2′‐(1‐Oxidopyridine‐2,6‐diyl)bis[4,5‐dihydro‐4,4,5,5‐tetramethyl‐3‐oxido‐1H‐imidazol‐1‐oxyl] Hydrate

The structure of a novel oxido‐aminoxyl (=`nitronyl nitroxide') biradical, 2,2′‐(1‐oxidopyridine‐2,6‐diyl)bis[4,5‐dihydro‐4,4,5,5‐tetramethyl‐3‐oxido‐1H‐imidazol‐1‐oxyl] hydrate ( 1 ⋅H₂O) was established by X‐ray analysis in the solid state: monoclinic, space group P2₁/c, Z=4 with a=12.621(4), b=15.704(5), and c=13.001(4) Å, and β=115.202(6)°. Variable‐temperature magnetic susceptibilities show a weak antiferromagnetic interaction between the two oxido‐substituted aminoxyl moieties of 1 , indicative of a singlet ground state. AM1 Calculations located minima for the possible structure based on the X‐ray crystal structure. A hybride density‐functional‐theory calculation with the UB3LYP method from the X‐ray crystal structure establishes the same spin sign in the two aminoxyl moieties and shows that a small spin density is localized at the C‐atoms of the pyridine moiety. These theoretic results are in good agreement with the determined weak antiferromagnetic interaction of 1 .     

Synthesis and Photochromic Behaviour of Novel 2H‐1‐Benzopyrans (=2H‐Chromenes) Derived from Carbazololes

The synthesis and photochromic properties of new 2,2‐diphenyl‐2H‐1‐benzopyrans, fused to an indole moiety, are described. All compounds exhibit photochromic behaviour in solution at room temperature. The heteroanellation effects are variable and depend on the position and geometry of the fused indole moiety. A general bathochromic shift in the spectra of the open forms is observed. The presence of a N‐methyl group prevents the broadening of the absorption spectra and promotes the instability of some photoinduced forms of compounds with the indole moiety fused at the 5,6 positions of the 2H‐1‐benzopyran skeleton. The enhanced photocolouration efficiency in the near‐UV and the kinetics of thermal bleaching indicate that the novel compounds with an indole moiety fused at the 6,7 positions, particularly those with a linked thiophene moiety, are very interesting molecules for applications in the field of variable optical absorption systems.     

Spiro[4.4]nonatetraene and its Positive and Negative Radical Ions: Molectronic Structure Investigations

The electronic structure of spiro[4.4]nonatetraene 1 as well as that of its radical anion and cation were studied by different spectroscopies. The electron‐energy‐loss spectrum in the gas phase revealed the lowest triplet state at 2.98 eV and a group of three overlapping triplet states in the 4.5 – 5.0 eV range, as well as a number of valence and Rydberg singlet excited states. Electron‐impact excitation functions of pure vibrational and triplet states identified various states of the negative ion, in particular the ground state with an attachment energy of 0.8 eV, an excited state corresponding to a temporary electron attachment to the 2b₁ MO at an attachment energy of 2.7 eV, and a core excited state at 4.0 eV. Electronic‐absorption spectroscopy in cryogenic matrices revealed several states of the positive ion, in particular a richly structured first band at 1.27 eV, and the first electronic transition of the radical anion. Vibrations of the ground state of the cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density‐functional and CASSCF/CASPT2 quantum‐chemical calculations. In their various forms, the calculations successfully rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule, the excitation energies of the radical cation, its IR spectrum, the vibrations excited in the first electronic absorption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized by a calculation of the Jahn‐Teller distortion of the anion ground state. Contrary to expectations based on a single‐configuration model for the electronic states of 1 , it is found that the gap between the first two excited states is different in the singlet and the triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.     

Photosensitized Generation of Singlet Oxygen from (Substituted Bipyridine)ruthenium(II) Complexes

Photophysical properties in dilute MeCN solution are reported for seven Ru^II complexes containing two 2,2′‐bipyridine (bpy) ligands and different third ligands, six of which contain a variety of 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridines, for one complex containing no 2,2′‐bipyridine, but 2 of these different ligands, for three multinuclear Ru^II complexes containing 2 or 4 [Ru(bpy)₂] moieties and also coordinated via 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridine ligands, and for the complex [(Ru(bpy)₂(L)]²⁺ where L is N,N′‐([2,2′‐bipyridine]‐4,4′‐diyl)bis[3‐methoxypropanamide]. Absorption maxima are red‐shifted with respect to [Ru(bpy)₃]²⁺, as are phosphorescence maxima which vary from 622 to 656 nm. The lifetimes of the lowest excited triplet metal‐to‐ligand charge transfer states ³MLCT in de‐aerated MeCN are equal to or longer than for [Ru(bpy)₃]²⁺ and vary considerably, i.e., from 0.86 to 1.71 μs. Rate constants k_q for quenching by O₂ of the ³MLCT states were measured and found to be well below diffusion‐controlled, ranging from 1.2 to 2.0⋅10⁹ dm³ mol⁻¹ s⁻¹. The efficiencies f of singlet‐oxygen formation during oxygen quenching of these ³MLCT states are relatively high, namely 0.53 – 0.89. The product of k_q and f gives the net rate constant k for quenching due to energy transfer to produce singlet oxygen, and k_q−k equals k, the net rate constant for quenching due to energy dissipation of the excited ³MLCT states without energy transfer. The quenching rate constants were both found to correlate with ΔG^CT, the free‐energy change for charge transfer from the excited Ru complex to oxygen, and the relative and absolute values of these rate constants are discussed.     

Ternary complexes in gels from agarose and from chemically‐modified agarose

Thermodynamic data and mechanical measurements are shown for gels prepared in aqueous binary solvents (water/DMSO, water/DMF, water/methyl formamide and water/formamide). When electrostatic interactions, as opposed to hydrogen bonding, can be established with the cosolvent (DMSO, DMF, methyl formamide) we come to the conclusion that ternary complexes are formed (agarose/water/cosolvent). In the case of chemically‐modified agarose (OH groups replaced by OCH₃ groups) we suggest that these cosolvents are directly involved in the formation of the gel.